The present invention relates to an anti torque shock control method and device for a power transmission system for a vehicle, and more particularly relates to such an anti torque shock control method and device, particularly applicable to a vehicle power transmission system in which a clutch is provided for selectively engaging two of the vehicle wheels together at least partially.
The present invention has been described in Japanese Patent Applications Ser. Nos. Showa 60-194634 (1985) and Showa 61-176468 (1986), both of which were filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. One type of such transmission system is the so called part time four wheel drive system, in which a 2WD/4WD control clutch is included which can switch the transmission operational mode between a two wheel drive operational mode and a four wheel drive operational mode, i.e. typically which can selectively power the front wheels of the vehicle from the engine while the rear wheels of the vehicle ae always being powered from said engine.
Also, there is currently sometimes provided a type of so called full time four wheel drive type of transmission which remains always engaged to four wheel drive without any episodes of two wheel driving, and this type is becoming more and more popular. In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front-rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking pnenomenon created by the different in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof. Also, it has been practiced to provide a device to such a front-rear differential device which prevents said front-rear differential device from performing differential action, in a selective fashion. When such a center differential action inhibition means, which typically may be a friction engaging means such as a hydraulic clutch, is actuated, it causes the differential action provided by said front-rear differential device between the front vehicle wheels and the rear vehicle wheels to be at least partially prevented, and instead said front vehicle wheels, considered as a pair, are driven from the vehicle engine, and also said rear vehicle wheels, considered as a pair, are at least partially independently driven from said vehicle engine. Such types of structure are at least partly disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 50-147027 (1975), Japanese Patent Application Laying Open Publication Ser. No. 55-72420 (1980), Japanese Patent Application Laying Open Publication Ser. No. 56-138020 (1981), and Japanese Utility Model Application Laying Open Publication Ser. No. 61-73430 (1986), none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law.
Also, there is a per se known type of so called limited slip differential device for a vehicle, not necessarily particularly associated with any four wheel drive transmission system, in which a differential device is provided for driving two vehicle wheels on the same vehicle axle with differential effect being provided therebetween, and in which a differential control clutch is provided to said differential device for selectively at least partially inhibiting said differential effect provided thereby.
Now, when a transmission for a vehicle is being used which has a fluid torque converter or a similar type of fluid coupling and also an auxiliary speed change mechanism such as an automatic transmission, the problem arises that, when the operating range of such a transmission is shifted from a non drive range such as "P" range or "N" range to a drive range such as "D" or "R" range, at this time drive force starts to be transmitted to certain vehicle drive wheels whereas before this was not the case, and this engenders a risk of slack in a differential device (either a front or a rear differential device, or a front/rear type of 4WD differential device) of the vehicle causing shift shock and so called "clonking", which not only is disconcerting and uncomfortable for passengers in the vehicle, but is liable to shorten the life of the transmission and/or the differential device, and to compromise their reliability.
Particularly with a four wheel drive type of vehicle, whether or not said be provided with a center differential device (i.e. a front/rear type of 4WD differential device), since it is typical in such a case for a rear differential device to be provided for providing differential effect between the two rear vehicle wheels and also for a front differential device to be provided for providing differential effect between the two front vehicle wheels, in this case when the operating range of the transmission is shifted from a non drive range such as "P" range or "N" range to a drive range such as "D" or "R" range, at this time one of these front and rear differential devices will typically have less slack and will take up drive first, whereupon the action of the central differential device will speed up the rotational motion of the other one of said front and rear differential devices, thus producing a relatively large shift shock or so called "clonking".
Further, in the event that such a front-rear differential device of the type described above is of an unequal distribution type which distributes drive torque substantially unequally between the front vehicle wheels and the rear vehicle wheels, then, when the operating range of the transmission is shifted from a non drive range such as "P" range or "N" range to a drive range such as "D" or "R" range, at this time the pair of vehicle wheels (typically the rear wheels) which receive more torque sink, or more properly the portion of the vehicle body above said pair of vehicle wheels drops, more than does the portion of the vehicle body above the other pair of vehicle wheels (typically the front wheels); and this causes the so called vehicle squat problem, which is troublesome and disconcerting for the vehicle passengers, and can lead to difficulties with vehicle steering and control. This problem further is accentuated in the case of a relatively soft vehicle suspension.